The present invention relates to fluid controllers, and more particularly, to such controllers which are used in load sensing hydraulic systems.
It will become apparent to those skilled in the art that the present invention may be used advantageously with any type of fluid controller which is operable to control the flow of fluid from a source of pressurized fluid to a fluid operated device, wherein the source of fluid includes pressure responsive means for varying the delivery of fluid to the controller. The present invention is especially useful when applied to controllers such as the steering control unit of a full fluid-linked vehicle steering system, and the invention will be described in connection therewith.
The use of load sensing controllers has become increasingly popular, partly because such controllers result in a constant fluid flow for a given rate of steering wheel rotation, regardless of steering load, and partly because such controllers save a substantial amount of energy, when compared to conventional open center and closed center controllers.
Load sensing controllers are normally used in a system of the type shown in U.S. Pat. No. 3,455,210, assigned to the assignee of the present invention. Such systems include a load sensing priority flow control valve which directs pump output flow to either the load sensing controller, which is the priority device, or an auxiliary load circuit, in response to variations in the steering load signal. Initially, the load signal used in such systems was a "static" load signal, i.e., there was no flow in the load signal circuit in the steady-state condition. Also, the load sensing systems initially commercialized used fixed displacement pumps, and because of the constant flow available from the fixed pump, the response time by the system to changes in the static load signal was generally satisfactory.
More recently, load sensing controllers are being used commercially in systems including load sensing pumps, i.e., the load signal from the controller is transmitted both to the load sensing priority flow control valve and to the load sensing pump. Because the load sensing pump delivers fluid only in response to a demand for fluid, and must increase pump stroke in order to do so, it has been found that system response time is not always satisfactory when using a static load signal, which is typically bled to the system reservoir when the controller is in neutral.
Partly to improve system response time, much work has been done by those skilled in the art with regard to the use of "dynamic" load signals, i.e., load signals wherein a small amount of fluid is pumped from the priority outlet port of the priority flow control valve, through the load signal circuit, and into the fluid controller where it recombines with the main flow path, downstream of the main variable flow control orifice. The use of a dynamic load signal has other advantages besides response time. For example, the dynamic signal, flowing toward the controller, permits the use of a check valve in the load signal line which prevents "wheel kick" by preventing the escape of fluid from the controller when steering against a volume of fluid which is trapped in the steering cylinder.
In the initial work with dynamic load signal systems, the controllers were the same as those which had been used with static load signals. One problem which resulted was the occurrence of "high transition pressures", i.e., when the controller valving was in transition between the neutral position and the fully actuated position, a substantial buildup of pressure would occur between the pump and the priority flow control valve. This pressure buildup would create an excessive load on both the pump and the vehicle engine, and the throttling of the high pressure fluid would create heat, wasting engine horsepower.